In a hybrid vehicle shown in JP-2000-179371A, an engine driving condition in which a fuel economy is optimal every demand power of the engine is established by an efficiency characteristic line which is defined by an engine speed and torque as parameters. On the efficiency characteristic line, an operation point (engine speed and engine torque) at which the demand power can be outputted is determined as a demand operation point. The engine is operated at the demand operation point.
The demand power of the engine is varied according to a demand vehicle driving force, a demand charging amount of a battery and the like. The operation point moves along an efficiency characteristic line according to the variation in demand power of the engine. Even if the operation point moves to deteriorate the fuel economy, the moving speed of the operation point is the same as the case where the operation point moves to improve the fuel economy, Hence, the fuel economy is deteriorated.